In order to protect a precision part (or an electronic device) against moisture, dust and other unfavorable substances, the precision part (such as a semiconductor element, a printed wiring board, or a solar cell) is sealed (or encapsulated) with a resin. As the sealing method, a known method of sealing a precision part comprises placing the precision part in a mold cavity and injecting a fluid resin into the cavity. This method uses a thermosetting resin having a low viscosity and a high flowability in many cases.
However, the thermosetting resin shortens a storage life due to an additive (such as a crosslinking agent) added to the thermosetting resin, and requires a relatively long time from injection of the resin into a mold cavity to curing of the resin. Thus, the thermosetting resin cannot achieve improvement of efficient production. Further, depending on the species of the resin, it is necessary to cure the resin after molding, which deteriorates production efficacy.
Moreover, it is known that a thermoplastic resin is injection-molded to seal a precision part. The thermoplastic resin, however, is practically injected at a relatively high temperature and high pressure, and thus a substrate or an electronic part mounted on a substrate is liable to damage and loses the reliability. Japanese Patent Application Laid-Open Publication No. 2000-133665 (JP-2000-133665A, Patent Document 1) discloses a method of sealing a printed wiring board mounted with an electronic part, the method comprising placing a printed wiring board equipped with an electronic part in a mold cavity and injecting a heat-melted polyamide resin having a temperature of 160 to 230° C. into the mold cavity at a pressure range of 2.5 to 25 kg/cm2. This document discloses in Examples that a polyamide resin (Series Number 187) manufactured by TRL (France) is injected into a mold at a melting temperature of 190° C. and a pressure of 20 kg/cm2 to seal a printed wiring board. This method, however, also exposes the electronic part to relatively high temperature and high pressure, and the electronic part is sometimes damaged.
Packing of a device by a film is also known. Japanese Patent Application Laid-Open Publication No. 2001-284779 (JP-2001-284779A, Patent Document 2) discloses a process for producing a sealed part of an electronic circuit; the process includes a method comprising inserting a circuit board (1) provided with electronic parts (2, 2A) into a cylindrical film (3) and closing both openings of the cylindrical film to pack the circuit board, a method comprising covering a region of the electronic parts with sheet films (3A, 3B), a method comprising covering both sides of a circuit board (1) with two sheet films (3A, 3B) and wrapping the circuit board with the sheet films. This document also discloses a method comprising covering the circuit board with film(s) previously softened by heat and a method comprising reducing a pressure between the board and the film(s) for following the film(s) to the above-mentioned part or board. Japanese Patent Application Laid-Open Publication No. 11-259021 (JP-11-259021A, Patent Document 3) discloses a liquid crystal display panel comprising liquid crystal display panel members consisting of a plurality of liquid crystal display panel members and a film, the panel members being covered and laminated with the film. This document also discloses a polyamide as an example of a plastic film.
In these methods, however, a device (such as an electronic part) cannot be sealed by tight adhesion. In particular, since these methods protect a device against moisture or water in a wrapping manner, the device cannot effectively be protected in covering only one side of the device.
Further, it is also known to seal a device using a film sealant. Japanese Patent Application Laid-Open Publication No. 2008-282906 (JP-2008-282906A, Patent Document 4) relates to a process for producing a solar cell module comprising a solar cell sealed between a substrate and a film by a resin; in the process, a first sealing-resin sheet substantially covering the whole surface of the substrate is disposed between the substrate and the solar cell, and a second sealing-resin sheet substantially covering the whole surface of the substrate is disposed between the film and the solar cell for preparing a layered structure. A plurality of the layered structures are stacked while a back plate is disposed outside the film of an uppermost layered structure, air between the substrate and the film is discharged and the resin is heat-melted and then cooled to seal the cell. This document discloses that the sealing-resin is selected from the group consisting of an ethylene-vinyl acetate copolymer, a poly(vinyl butyral), and a polyurethane.
Japanese Patent Application Laid-Open Publication No. 2009-99417 (JP-2009-99417A, Patent Document 5) discloses an organic electronic device sealing panel which comprises a substrate, an organic electronic device formed on the substrate, and a barrier film sealing the organic electronic device, and a hot-melt member is disposed between the organic electronic device and the barrier film. This document also discloses that the hot-melt member contains a moisture scavenger and a wax and that the hot-melt member is in a thin film having a thickness of not more than 100 μm. Moreover, Japanese Patent Application Laid-Open Publication No. 2009-99805 (JP-2009-99805A, Patent Document 6) discloses a hot-melt member for an organic thin-film solar cell, the member containing a moisture scavenger and a wax. This document also discloses that the hot-melt member may be in the form of a thin-film, a plate, an amorphous or indefinite, and others.
The film sealant, however, has low adaptability to an uneven portion (a depressed or raised portion) of a device, and thus it is difficult to seal the detailed exact or minutiae form of the device tightly. Further, since the above hot-melt member comprises a wax as a main component, it is difficult to seal the device with higher adhesion.
Japanese Patent Application Laid-Open Publication No. 2001-234125 (JP-2001-234125A, Patent Document 7) discloses a powder coating material for thermal spray coating; in order to prevent the coating material from discoloring even when exposed to high-temperature flames in a coating process, the coating material comprises 0.05 to 2.0 parts by weight of a hindered phenol-series antioxidant and 0.05 to 2.0 parts by weight of a phosphite-series antioxidant relative to 100 parts by weight of a thermoplastic resin, and has a medium particle diameter of 50 to 300 μm, a bulk specific gravity of not less than 0.30 g/ml and a repose angle of not more than 35°. In this document, the thermoplastic resin includes a polyethylene resin, a polypropylene resin, a nylon-11 resin, a nylon-12 resin, an ethylene-vinyl acetate copolymer resin, an ethylene-acrylic acid copolymer resin, an ethylene-methacrylic acid copolymer resin, a modified polyethylene resin, and a modified polypropylene resin. An example using a nylon (polyamide) resin (trade name “Grilamid” manufactured by EMS-CHEMIE AG) is also described in this document.
Since the above-mentioned powder coating material, however, is melted and sprayed at a high temperature, there is a possibility to easily damage the electronic part in the sealing process and the reliability of the device.